Problem: Immunization is one of the most effective public health interventions. Yet, nationally coverage has consistently fallen short of national goals, and has remained for the most part stagnant. The continued presence of vaccine-preventable diseases poses a threat to public health. In addition to needed improvement of immunization coverage for the general pediatric/adolescent population, some children with chronic medical conditions need specific additional immunizations, yet many fail to receive them. Immunization reminders for providers in the electronic health record (EHR) are a type of clinical decision support (CDS) that can reduce missed immunization opportunities. One limitation of these reminders is that they generally depend only on data local to the EHR, which can be incomplete due to record scatter, leading to inaccurate alerts. Another challenge is that pediatric and adolescent immunization schedules are complex. Thus, design, testing, and maintenance of forecasting rules, i.e., when each vaccine is due, is time-consuming. Thirdly, most immunization CDS systems do not take into account an individual child's medical conditions. An Immunization Information System (IIS), also known as an immunization registry, is a population- based system that collects immunization data primarily for children and adolescents from providers at a regional or state level. In most cases, access to IIS data and forecasting tools are available only by visiting the IIS's website. However, frontline care providers are most likely to benefit when IIS provide them with data and forecasting at point of patient care in the form of a reminder within their current EHR workflow. In this grant, we seek to couple bidirectional exchange of IIS immunization information and forecasting tools with patient level medical history from the EHR to deliver accurate, patient-specific EHR immunization reminders. Building on our two previous AHRQ-funded projects, the proposed work will generate empiric knowledge regarding the best practices for implementing immunization reminders and computerized CDS in general, including capitalizing on data-sharing with public health entities like an IIS. AIM 1: Assess the impact of EHR reminders integrated with immunization data and forecasting from a regional IIS on receipt of generally recommended immunizations in a low-income, urban, pediatric and adolescent population; AIM 2: Assess the impact of integrated EHR reminders that also incorporate patient's medical conditions on receipt of immunizations specifically recommended for children and adolescents with chronic medical conditions. Methods: We will conduct a randomized cluster crossover pragmatic clinical trial to assess immunization outcomes during periods when a reminder is on vs. off. The records of >12,000 children will be reviewed. Outcomes: Outcomes include under-immunization, over-immunization, and captured opportunities according to the CDC-recommended age-specific as well as medical condition-specific immunizations.